Low profile operating switch panels are used in smaller instruments, such as mobile telephones. Dome switches have been used for such applications and exhibit snap ratios and click characteristics providing desired tactile feedback to the user. Such attributes have been mitigated, however, by the need to isolate the panel circuitry from outside contaminants including humidity and particulates. A typical approach is disclosed in U.S. Pat. No. 6,917,007 wherein the metal dome switches are hermetically encased in composite carrier and adhesive layers. As a result of the composite structure, the flex characteristics of the dome, and accordingly the snap ratio are altered and tactile feedback mitigated, slightly but adversely from a user perspective.
The snap ratio, also referred to as tactile feel, is the response to an operator depressing a keypad and feeling a quick force drop (or click) at their fingertip. Generally, the snap ratio for a dome type switch is represented as shown below:
      Snap    ⁢                  ⁢    ratio    ⁢                  ⁢          (      %      )        =                    (                  OF          -          RF                )            OF        ×    100  
wherein, OF: (Operational Force) is the maximum value of load necessary for deforming the dome shape switch from open position dome shape to flexed state engaging fixed contacts in the closed position: and RF: (Recovery Force) is the value of resistive load at contact closed position. The desired snap ratio is reached when the result of the calculation is 50%. FIG. 7 is a graph illustrating the snap ratio for a single isolated dome (solid lines), for a typical encased prior art dome (dashed lines), and for a metal dome type switch according to the invention (dotted lines) and as described below. The ordinate designates load, and the abscissa designates operational distance. A large force OF is needed during travel after starting to depress the apex portion. However, when the apex portion is recessed to some degree, the necessary load is reduced and a small RF is sufficient to close the fixed contacts
Keypads with snap ratios of 50% have excellent tactile feel and relatively long life. Keypads with snap ratios below 40% have relatively weak tactile feel, yet longer life. It can be determined from the above formula that snap ratio depends upon the variance between Operational Force (OF) and the Recovery Force (RF). This value will directly influence tactile feel, which is caused by force variation. That is, when the force drops from OF to RF, the operator can feel a force change at their fingertip immediately. There have been prior attempts to modify metal dome type switches to improve snap ratio as disclosed by example, U.S. Pat. No. 6,595,653 to Atsushi Saito wherein cut portions are provided along an outer peripheral edge of the dome shaped movable contacts. One shortcoming of this invention is that it includes cuts through the EL (electro luminescent) film to achieve an improved snap ratio. Also, under stress, the switch actuation can shear the adhesive interface between the dome contact and the cuts thereby compromising the hermetic sealing.